1. Field of the Invention
The present invention relates to an electromagnetic clutch for a compressor, and more particularly, to a field coil assembly having a coil made by winding an aluminum wire therein.
2. Description of the Related Art
Generally, an electromagnetic clutch for a compressor is an electric device, wherein a magnetic field is formed by electromagnetic induction of a coil made by winding a wire when power is supplied thereto and a resultant magnetic force causes a disc at a driving shaft of a compressor to be attracted toward a frictional surface of a pulley and to be dynamically connected thereto, whereby driving force of the pulley rotated by an engine is transmitted to the hub disc of the driving shaft of the compressor. The electromagnetic clutch for a compressor serves to control the operation of a cooling system of an air conditioner by regulating power of the compressor according to whether or not power is applied to the coil.
FIG. 1 is an exploded perspective view showing a general compressor and a coil assembly of an electromagnetic clutch for a compressor.
According to the figure, a general compressor includes a compressor body 1 for compressing and discharging a refrigerant, a pulley 3 connected to an engine and rotated, a field coil assembly 2 positioned between the pulley 3 and the compressor body 1 to generate electromagnetic force according to power supplied thereto, and a hub 4 connected to a driving shaft of the compressor body 1 and coupled to the pulley 3 by the electromagnetic force of the field coil assembly 2.
The compressor as configured above connects or disconnects the pulley 3 and the hub 4 using the power supplied to the field coil assembly 2 according to the manipulation of a user, thereby controlling power transmission of the compressor body 1.
If power is applied to the field coil assembly 2, an electric field made by a coil 2d of the field coil assembly 2 generates electromagnetic force, and the hub 4 is moved in an axial direction by the electromagnetic force and thus comes into contact with the pulley 3. Thereafter, the hub 4 and the pulley 3 rotate together by means of frictional force therebetween, so that rotational force of the pulley 3 is transmitted to the hub 4.
Accordingly, the compressor body 1 whose the driving shaft is coupled to the hub 4 operates, and then, a refrigerant is compressed and discharged as the compressor body 1 operates. If the power applied to the field coil assembly 2 is cut off, the electromagnetic force is not generated. Thus, the hub 4 and the pulley 3 are disconnected from each other, whereby the compressor body also stops.
The conventional field coil assembly 2 of an electromagnetic clutch for a compressor has its external appearance and framework defined by a core 2a, as shown in the figure. A connector 2b is provided on the core 2a to give an electric connection to the outside and also be supplied with power.
In addition, a bobbin 2c is provided in the core 2a, and the bobbin 2c is provided with a terminal 2c′ connected to the connector 2b. 
The bobbin 2c is provided with a coil 2d. The coil 2d is formed by winding a wire, wherein lead wires (not shown) for connecting with the terminal 2c′ are provided at one side of the coil.
The bobbin 2c is provided with a fixing cover 2f. The fixing cover 2f is coupled to the bobbin 2c to support the coil 2d. 
However, the above related art has the following problems.
The wire is generally made of a copper material. However, copper has a relatively high specific weight of 8.96, which increases the entire weight of the field coil assembly 2, thereby deteriorating a fuel efficiency of a vehicle. Also, copper is relatively expensive, thereby increasing a production cost of the field coil assembly 2.
To solve this problem, the wire can be made of aluminum with a relatively low specific weight (2.7). However, since aluminum has a relatively higher inherent resistance than copper, when an aluminum wire with the same diameter is used, the efficiency of the field coil assembly 2 is deteriorated and the coil may be easily heated.